Methods and apparatus for generating two phase-coherent signals with arbitrary frequency ratio have application in a variety of devices, particularly phase-coherent transponders. Phase-coherent transponders receive a radio frequency (RF) signal on one frequency and transmit a signal on a different frequency in response. The input and output frequencies of the transponder are phase-coherent and are related by a turn-around ratio, also known as the coherency ratio. Typical technology to generate two phase-coherent signals requires elaborate multiplier/divider chains to generate the signals. Such technology is not flexible, and requires a unique hardware design for each frequency ratio desired. A single design does not provide for the generation of arbitrary frequency ratios.
Multiplier/divider technology achieves coherency of the input and output signals by repeatedly multiplying and dividing an input signal (e.g. multiplication by two followed by division by 3, etc.) to generate the output signals. Such implementations require complex hardware, with a corresponding large number of gates, cost, power, and area, especially if the desired coherency ratio M/N is the quotient of two relatively large numbers, e.g. 256/205.
Other methods and apparatus for generating two phase-coherent signals of differing frequency ratio involve changing coherency ratios over time, without feedback, or using different reference clocks. The method of changing coherency ratios over time requires an arithmetic processor, however, and using different reference clocks is valid only for one frequency ratio.
Thus, what is needed is an apparatus and method which is flexible and provides for arbitrary frequency ratios of the generated two phase-coherent signals within a single design. It is desirable if the arbitrary frequency ratio can be programmed without hardware changes.